1. Field of the Invention
The present invention relates to an image forming apparatus which forms a color image by the steps of forming a latent image, e.g., an electrostatic latent image, for each of colors, developing the latent image through use of each color toner to thereby produce a monochrome image, superimposing the monochrome images on each other on predetermined paper, and fixing them, and also relates to an image forming method for use with the image forming apparatus. More particularly, the present invention relates to an image forming apparatus which forms a color image by adoption of an image forming method such as an electrophotographic recording method, an electrostatic recording method, ionography, or a magnetic recording method, and also relates to an image forming method for use with the apparatus.
2. Description of the Related Art
Image forming apparatuses which form a color image have widely been in actual use in recent years. Particularly, a color image forming apparatus equipped with a plurality of image carriers has been developed taking full advantage of its superior productivity as a rival to an existing color image forming apparatus which creates one copy by a plurality of rotations (e.g., four rotations). In the color image forming apparatus equipped with a plurality of image carriers (hereinafter referred to as a tandem-type image forming apparatus), the plurality of image carriers, e.g., a plurality of photosensitive members in the case of an electrophotographic color image forming apparatus, are likely to be arranged substantially linearly. For this reason, a charging device, an exposure device, a developing device, and a cleaner, all of which are disposed around each photosensitive member, can be designed so as to have substantially the same construction in common according to the type of the device. Accordingly, the tandem-type image forming apparatus has the advantages of compactness, a low price, and high reliability. Conversely, the apparatus has the disadvantage of the extremely difficulty of registration of color images which is the most important factor in determining color picture quality.
FIG. 21 shows one example of the construction of an existing color image forming apparatus which comprises four photosensitive members 1, 2, 3, 4 and a belt-like transfer member 5 extended across them. A charging device 11, an exposure device 12, a developing device 13, and a cleaner 14 are disposed around the photosensitive member 1; a charging device 21, an exposure device 22, a developing device 23, and a cleaner 24 are disposed around the photosensitive member 2; a charging device 31, an exposure device 32, a developing device 33, and a cleaner 34 are disposed around the photosensitive member 3; and a charging device 41, an exposure device 42, a developing device 43, and a cleaner 44 are disposed around the photosensitive member 4. The belt-like transfer member 5 may be formed into a so-called intermediate transfer member on which toner images are directly placed or into a so-called transfer paper carrier in which toner images are transferred onto the paper sucked by the belt-like transfer member 5.
Next, with reference to the case of the intermediate transfer belt, the formation of an image is described in detail. To begin with, after having been evenly electrostatically charged by the charging device 11, the photosensitive member 1 is exposed to light by the exposure device 12, whereby an electrostatic latent image is formed. The thus-formed latent image is developed and visualized through use of monochrome toner to thereby produce a toner image. The toner image, or the first toner image, is then transferred to the intermediate transfer belt 5 at the position where the toner image comes into contact with the intermediate transfer belt 5. Another color toner image which is similarly formed on the photosensitive member 2 in synchronization with the travel of the first toner image to the position where the first toner image comes into contact with the photosensitive material 2 is superimposed as the second toner image on the first toner image. Similarly, the third and fourth toner images are further superimposed on the thus-superimposed toner images to thereby 4-color superimposed images. These images are transferred as a single image to and fixed on paper (not shown), whereby a color image is formed on the paper.
While passing by the positions where the intermediate transfer belt 5 comes into contact with the respective four photosensitive members 1, 2, 3, and 4, the intermediate transfer belt 5 does not successfully travel along the positions, because it extends/contracts or snakes it's way by dint of the tensile force stemmed from the driving of the intermediate transfer belt 5. As a result, the color toner images become slightly offset from each other, thereby resulting in an undesirable image. Various methods have already been adopted to position the toner images. With one of the methods, a predetermined image is developed, and the position of this developed image is read by an image detection sensor. After the positions of respective color images have been calculated, for example, the starting point for writing operations of the laser light source or the position of a reflection mirror of the light source is finely adjusted. However, this methods requires a very complicated mechanism, thereby resulting in an inevitable increase in the cost.
There is proposed another method in which a plurality of photosensitive members are arranged around a larger cylindrical roll- or drum-shaped rotary transfer member for purposes of preventing dislocation. This method is illustrated in FIG. 22. A rotary transfer member 51 may be an intermediate transfer member or a paper carrier. For the roll- or drum-shaped rotary transfer member 51, it does neither extend/contract nor snake in contrast with the belt. Hence, the rotary transfer member 51 rotates around a drive shaft in a very stable manner, thereby rendering the mutual registration of images comparatively easy. In connection with this method, there is a more desirable method disclosed in Utility Model Publication (Kokoku) No. Hei-6-18364, by which the surface velocity of each of the photosensitive members (image carriers) comparatively easily differs from the surface velocity of the rotary transfer member. The difference in surface velocity between the photosensitive members and the rotary transfer member is intended to prevent the rotary transfer member from receiving a different speed from each of the four photosensitive members while following the rotations of the photosensitive members. The rotary transfer member has the advantage of easy speed control. However, as can be seen from FIG. 22, there is the need for the use of a rotary transfer member having a very large diameter. Further, components to be provided around the respective photosensitive members 1, 2, 3, 4, e.g., the charging devices 11, 21, 31, 41, the exposure devices 12, 22, 32, 42, the developing devices 13, 23, 33, 43, and the cleaners 14, 24, 34, 44 are different in position from each other with reference to the direction of gravity. Therefore, it is impossible to achieve the commonality of components, thereby posing serious problems with regard to cost, reliability, or size. Although it is possible to use a member having a smaller diameter for the rotary transfer member 51, the difficulty of achieving the commonality of constituent components to be provided around the photosensitive member is multiplied greatly.